This invention relates to the field of radio transmissions systems and, more particularly, to a system for providing all of the advantages of present FM systems within one-half the normal bandwidth.
It is well known that angular modulation systems have advantages over amplitude modulation systems, notably in the areas of signal-to-noise ratio and in the ability to "capture" a receiver. However, due to the steadily increasing demand for more communication channels throughout the radio spectrum, the smaller bandwidth required by AM-SSB systems has begun to appear more desirable in spite of the considerable disadvantages. Thus, there has developed a need for a system having the advantages of FM within the bandwidth of an AM-SSB system.
Known angular modulation systems are of two main types. One system uses direct frequency modulation of an oscillator which could not be a stable oscillator such as a crystal oscillator except with a very low modulation index. The usual phase modulated system uses a stable oscillator with narrowband modulation, then multiplies the frequency of the modulated signal to obtain the desired broadband signal.